A semiconductor device, such as a transistor, an IC, and an LSI, is encapsulated (sealed) in a package, such as a plastic seal package, for protection from the outer environment and easy handling. Such a package typically includes a dual inline package (DIP). DIP is a pin insertion type package, which is mounted on a substrate by inserting pins into through-holes of the substrate.
In recent years, the integration density and speed of semiconductor devices such as LSI chips have been increasing. Not only with the technological advancements but with an increasing demand for small-sized electronic parts to have high performance, the pin insertion type packages such as DIP have been displaced with flat packages for surface mounting. In the packages for surface mounting, pins are led out flatly and directly bonded to the surface of a substrate by soldering. Such semiconductor devices for surface mounting enjoy thinness, lightness, and smallness and are therefore advantageous in that the area ratio on the substrate is so reduced and that they may be mounted on both sides of a substrate.
On the other hand, the integration density of semiconductor devices, which is increasing year by year as is typically seen in memory devices and micro processors, has been achieved through advanced techniques of fine patterning and increase of the chip scale. This leads to confrontation between size reduction of a seal package and size increase of a chip. As a result, the plastic seal part has been necessarily made thinner.
Where the package for surface mounting contains moisture before surface mounting, there is a problem that the moisture is vaporized on soldering to cause package cracking. As shown in FIG. 1, the moisture passes through sealing resin 1 and enters into the inside of package 3 as indicated by arrow A and remains mostly on the surface of silicon chip 7 or on the back side of die bonding pad 4. When the package is mounted on a substrate by soldering, for example, vapor phase soldering, the remaining moisture is vaporized by the heat of soldering. As a result, the vapor pressure pushes down the resin part on the back side of die bonding pad 4 to make gap 5 and, at the same time, to cause crack 6 in package 3 as shown in FIG. 2. In FIGS. 1 and 2, numeral 8 indicates a bonding wire.
In order to overcome this problem, it has been proposed to put the whole device, i.e., the package, under moistureproof seal until it is surface mounted. It has also been proposed that the package is dried at 100.degree. C. for 24 hours for moisture exclusion immediately before surface mounting by soldering. While these proposals have already been put to practice, such prehandling requires time and labor.
On the other hand, to make a semiconductor device larger is accompanied by problems attributed to thermal stresses, such as passivation layer crack and deformation of an aluminum pattern. These problems have been overcome usually by using a resin composition having dispersed therein a soft substance, e.g., rubber or silicone, as a sealing resin thereby to reduce the stress. Such a means being taken, however, the moisture diffusion coefficient of the sealing resin increases, which is disadvantageous from the standpoint of prevention of package cracking on soldering.